This invention relates to instruments for weighing a section of belt conveyor,and more particularly to so-called belt conveyor weighbridges having load sensing assemblies for producing electrical signals representative of the weight of a section of belt conveyor.
Reference is made to U.S. Pat. No. 4,682,664 disclosing a prior art weighbridge for a conveyor belt assembly. This weighbridge consists of a first cross beam supporting a plurality of idle rolls for carrying a portion of the conveyor belt and the material transported by the belt. It further includes a second cross beam beneath the first which is mounted by the belt. It further includes a second cross beam beneath the first which is mounted on conveyer frame members positioned alongside the conveyor belt for supporting the weighbridge. A pair of load cells on the second crossbeam support the first crossbeam and measure the weight of the desired portion of the conveyor belt.
One problem present in the installation of the prior art weighbridges of the type such as described in U.S. Pat. No. 4,682,664 is that because of manufacturing, fabricating and installation tolerances, misalignment and improper positioning of the weighbridge relative to the axis of movement of the conveyor belt can and often does occur. This results in forces acting both longitudinally and transversely relative to the axis of the direction of motion of the conveyor belt acting upon the weighbridge and inducing stresses in the load sensing assembly that contribute to the load measured by the load sensing assembly. When this occurs the load sensing assembly generates a signal which is intended to be solely representative of the normal force component (e.g., the downward vertical force component for a horizontal conveyor belt) of a portion of the conveyor belt and the material thereon but which in face is representative not only of that downward force but of also of the stress induced by the these lateral forces created on installation of the weighbridge. The cantilevered construction of the load cell illustated in FIG. 2 of U.S. Pat. No. 4,682,664 is designed to enhance vertical resolution of the loads applied to the weighbridge and thus eliminate signals arising from lateral forces. However, so long as such induced forces are present they will contribute at least to some extent to the signals generated by the load cells.
A further problem with prior art weighbridge devices of the type such as shown for example in U.S. Pat. 4,682,664 is that such weighbridges must be designed and manufactured for conveyor belt assemblies of a single specified width. More particularly, a weighbridge built in accordance with the complex desig illustrated in FIG. 1 of U.S. Pat. No. 4,682,664 is of a specified length which can then be accommodated only into a conveyor belt assembly having frame members that will accommodate this particular weighbridge length. Moreover, such weighbridge can support only a limited range of conveyor belt widths. If a conveyor belt assembly having frame members presenting a different width are utilized, then the weighbridge must then be made of another and different length to be useable in that assembly.